Method and apparatus for isolating a susceptor heating element from a chemical vapor deposition environment

ABSTRACT

An apparatus for efficiently heating a wafer within a CVD environment isolates the heating element of the apparatus from the CVD environment and includes a susceptor body defining a sealed space therein for containing the heating element and a surface coupled to the heating element for supporting and heating the wafer. The susceptor space is sealed from the CVD environment and is vacuumed to a first pressure. Heating gas is delivered through a space extending through the susceptor body which is sealed from the susceptor space containing the heating element and is vacuumed to a second pressure which is preferably less than the CVD reaction pressure to vacuum clamp a wafer to the susceptor. The heating gas delivery space is formed by an elongated sheath surrounding a hollow wafer lift tube and the sheath is sealed at one end to the backplane of the susceptor and at the other end to the tube. The wafer lift tube moves up and down within the sheath to lift a wafer. Various unique seals provide isolation of the heating element space and gas delivery space from each other and from the CVD reaction environment to protect the heating elements from the corrosive effects of CVD vapors.

FIELD OF THE INVENTION

This application relates generally to a heated semiconductor waferprocessing susceptor which utilizes backside gas to facilitate heattransfer to the wafer in a chemical vapor deposition ("CVD") reactionchamber, and more particularly to an apparatus and method for isolatingand protecting the susceptor heater element from the CVD environment.

BACKGROUND OF THE INVENTION

Semiconductor wafers are subject to a variety of processing steps in thecourse of the manufacture of semiconductor devices. The processing stepsare usually carried out in a sealed vacuum chamber of a wafer processingsystem. One such group of processing steps is referred to collectivelyas chemical vapor deposition or CVD. CVD encompasses deposition ofmaterial layers onto a wafer by a reaction of various chemicals whichare usually in a gaseous or vapor state. However, CVD processes mightalso involve chemical etching of the wafer. Various of the chemicalgases and vapors used in CVD process are corrosive to some of the metalparts and devices utilized in the reaction chamber of a wafer processingsystem. Accordingly, it is necessary to isolate and protect such partsand devices from the chemical gases.

CVD processes generally involve heating the wafer prior to deposition oretching. CVD is typically performed in a cold wall reactor, where thewafers to be coated or etched are heated to a reaction temperature on awafer-supporting susceptor while other surfaces of the reactor aremaintained at sub-reaction temperatures to prevent the deposition offilms thereon. For tungsten chemical vapor deposition, for example, thewafer is heated while the reactor walls are cooled, often to aroundambient room temperature. Alternatively, for titanium nitride chemicalvapor deposition, the walls may be heated above ambient roomtemperature, but to a temperature below that of the wafer being coated.

Heating substrates in a chemical vapor deposition environment within areactor is often difficult because the heating elements used to heat thewafer must be isolated from the corrosive chemical vapors and gasesutilized in the CVD process. One possible solution involves the use ofinfrared (IR) radiation to heat the wafer which is directed onto thewafer from outside the reaction chamber through IR windows formed in thereactor. However, temperature uniformity is often difficult to achievewith IR radiation, because the IR windows become coated with materiallayers by the CVD process leading to inconsistent heating of the wafer.Alternatively, resistive or resistance-type heaters coupled to thewafer-supporting susceptor within the reactor have provided bettertemperature uniformity and stability.

Generally, resistance-type heaters are mounted within a wafer-supportingsusceptor structure to a susceptor backplane or platen and operate toheat the susceptor backplane and simultaneously heat the wafer to thedesired operating temperature. However, since resistance-type heatingelements must heat the susceptor backplane while heating the wafer, thethermal response time may be slower than is desired to achieve maximumthroughput of the wafers. Therefore, to improve the thermal responsetime, a gaseous medium, such as helium, is pumped between the susceptorbackplane surface and the wafer supported on the backplane whichenhances the transfer of heat between the susceptor backplane and wafer.

A typical semiconductor wafer-supporting susceptor provided within areaction chamber has fixed to its bottom a susceptor drive supportframe. Rotatably mounted within the drive support frame is a hollowsusceptor drive shaft which is utilized to rotate the susceptor, whendesired, during CVD processing. The hollow susceptor drive shaft isrigidly connected to the bottom of the susceptor. A hollow space withinthe drive shaft communicates with the interior of the susceptor insidethe reaction chamber. Penetrations and openings are made in thesusceptor and susceptor backplane so that a vacuum hold of the wafer maybe accomplished. Ordinarily, the vacuum pressure inside the hollow driveshaft and within the susceptor interior is maintained at a pressuresufficiently lower than that of the reaction chamber to develop a vacuumwithin the susceptor which operates as a vacuum chuck to hold a waferagainst the heated susceptor backplane during processing. However, thepenetrations in the susceptor and susceptor backplane, as well as theinterface between the drive shaft and susceptor, provide paths forcorrosive chemicals to enter the interior of the susceptor and come intocontact with the heating elements. As a result, the heating elements maybe damaged, especially whenever a wafer is not held to the susceptorbackplane under vacuum to block the backplane penetrations and openings.

Furthermore, penetrations through the susceptor and backplane must alsobe made to provide a passage for the heating gas to reach the backsideof the wafer. Again, the susceptor heating elements are somewhatisolated from the corrosive chemical vapors as long as the penetrationsare covered by a wafer, but when the wafer is not present, there is adirect path for corrosive gases from the reaction chamber to the heatingelements through the heating gas penetrations.

Accordingly, there is a need for a susceptor or similar wafer-supportingdevice for use in a CVD environment which provides adequate isolationand protection of the heating elements from the corrosive chemicalvapors both during processing of a wafer and upon removal of the wafer.There is further a need for such a device which isolates the heatingelements while providing satisfactory heating of the wafer as well assupplying backside heating gas to the wafer for more efficient heattransfer.

SUMMARY OF THE INVENTION

To address the needs within the prior art, the present inventionprovides a susceptor/heater assembly which utilizes two separate vacuumenvironments to effectively isolate resistive heating elements fromcorrosive chemical vapors utilized in CVD processing.

In accordance with the principles of the present invention, a susceptorlocated inside a CVD reaction chamber includes a body which has formedtherein a heater housing for supporting and containing one or moreresistive heating elements. The heating element is fixed to a susceptorbackplane mounted to the body which heats and supports a wafer duringCVD processing. The heater housing formed within the susceptor body isevacuated to create a first vacuum environment. The first or outervacuum environment is maintained at a pressure of approximately 1 Torrto 100 Torr, and preferably around 25 Torr. The pressure within thefirst vacuum environment is also preferably maintained slightly abovethe process pressure within the reaction chamber. As a result, anyleakage between the reaction chamber and the first vacuum environment inthe heater housing will be out of the heater housing and into thereaction chamber rather than into the heater housing. The first vacuumenvironment is sealed from the CVD reaction chamber to prevent leakagein accordance with the objectives of the invention.

More specifically, the first vacuum environment is formed by sealing theheater housing with two high temperature metal seals. The seals preventthe corrosive gases and vapors of the reaction chamber from entering theheater housing and damaging the resistive heating elements and maintaina sealed environment which withstands the high temperatures of CVDprocessing. The first seal is maintained between the susceptor body andthe backplane structure rigidly mounted to the top of the susceptorbody. The first seal is preferably a silver compression seal whichextends annularly around the susceptor body and backplane interface toprevent leakage of the reaction chamber environment into the heaterhousing environment at that interface.

A composite metal second seal is utilized between the bottom of thesusceptor body and its interface with a drive shaft which rotates thesusceptor and provides a vacuum of the heater housing. The second sealhas a core with a unique diamond-shaped cross-section which ispreferably stainless steel. A ductile metallic coating, preferablysilver, surrounds the core and is deformed to accommodate surfaceimperfections between the susceptor body and drive shaft. The compositemetal seal is dimensioned and formed to withstand a large number ofthermal cycles without affecting the integrity of the seal.

Backside heating gas is delivered through a hollow wafer lift tube whichextends through openings within the susceptor body and backplane andconnects to a movable wafer support. The lift tube provides gas, such ashelium, between the wafer and backplane for efficient thermalconductivity between the resistive heating elements, the susceptorbackplane, and the wafer. The lift tube is vertically movable to lift awafer on the backplane and break the vacuum clamping hold and extendsgenerally concentrically inside the susceptor drive shaft. A second orinner vacuum environment is maintained around a portion of the lift tubeproximate the backplane.

Specifically, the lift tube which carries the backside heating gasextends through the center of the susceptor body and backplane. The lifttube is surrounded along part of its length by a sheath which defines aninner vacuum space. The lift tube is also coupled to a vacuum systemwhich evacuates the inside of the lift tube and the inner vacuum spaceof the sheath to define the second vacuum environment. The vacuum spaceof the sheath is coupled to the back of the wafer through an opening inthe susceptor backplane. The second vacuum environment through which thebackside heating gas travels is maintained at a pressure ofapproximately 1 Torr to 100 Torr which is dependent upon the processingpressure within the reaction chamber. Preferably, the pressure in thesecond vacuum environment is maintained at about 10 Torr. The pressurein the second vacuum environment is generally lower than processpressure maintained within the reaction chamber to maintain a vacuumholddown of the wafer onto the susceptor backplane during processing.When no wafer is present, the second vacuum environment and the backsidegas delivery system are maintained at the processing pressure of thereaction chamber.

To prevent the entry of corrosive chemical vapors into the heaterhousing through the second vacuum environment and backside gas deliverysystem, the second vacuum environment is isolated from the first vacuumenvironment by several seals. An upper seal is maintained between theinner face of the susceptor backplane and the sheath which surrounds thelift tube. The upper seal is preferably a nickel compression seal whichis formed of 100% nickel and is configured to withstand high CVDprocessing temperatures without compromising the seal. A lower seal isutilized between the sheath and the lift tube. The lower seal of thesecond vacuum environment is preferably a dynamic seal through which thelift tube may move as it travels vertically to lift the wafer away fromthe backplane. Alternatively, the upper seal might be eliminated bywelding the sheath to the backplane.

Therefore, during CVD processing, three vacuum environments exist withinthe CVD reaction chamber. The first vacuum environment within the heaterhousing surrounding the resistive heating elements is isolated from boththe reaction chamber environment and the backside gas delivery secondvacuum environment extending through the center of the heater housing.The pressure within the first vacuum environment is maintained higherthan the process pressure within the reaction chamber to prevent theleakage of corrosive chemical vapors into the heater housing. When awafer is being processed, the pressure within the second vacuumenvironment is lower than the reaction chamber pressure to insure avacuum holddown of the wafer. When there is no wafer present, there isdirect communication between the reaction chamber environment and thesecond or second or backside gas vacuum environment. However, this isnot harmful because the second vacuum environment is sealed from theheater housing first vacuum environment, and therefore, no corrosivechemical vapors can pass through the backside gas delivery system intothe heater housing and into contact with the resistive heating elements.As a result, the various seals and multiple vacuum environments provideconstant isolation of the heater housing environment and resistiveheating elements from the chemical vapors present in the reactionchamber environment. Thus, the heating elements are protected while thebackside gas is delivered to the backside of the wafer to facilitate amore efficient heat transfer between the heating elements and the wafer.

These and other objectives and advantages of the present invention willbecome more readily apparent in the following Detailed Description ofthe Invention taken in conjunction with the drawings herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view of a CVD module for a wafer processingdevice embodying the principles of the present invention;

FIG. 2 is a cross-sectional view of a CVD reactor of the module of FIG.1;

FIG. 3 is a cross-sectional view of the lower part of the reactor ofFIG. 2 illustrating the susceptor rotation and wafer lifting sectionsand various vacuum and gas connections;

FIG. 3A is a cross-sectional view taken along line 3A--3A of FIG. 3;

FIG. 4 is a cross-sectional view of a susceptor used in the CVD reactorshown in FIGS. 1 and 2, and incorporating the principles of the presentinvention;

FIG. 4A is an enlarged cross-sectional view of an encircled portion 4Aof FIG. 4 showing a sealing structure;

FIG. 5 is an enlarged cross-sectional view of portions of FIG. 4 showingadditional sealing structures of the present invention;

FIG. 5A is an enlarged cross-sectional view of a portion of FIG. 4showing the sheath welded to the backplane;

FIG, 6 is a cross-sectional view of another one of the sealingstructures utilized in the susceptor of FIG. 4;

FIG. 7 is a cross-sectional view of an alternative embodiment for asealing structure of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a CVD module 10 for a wafer processing cluster tool.Module 10 is the same as that shown in U.S. Pat. No. 5,273,588 issued onDec. 28, 1993 and assigned to the assignee of the present invention, theentire disclosure of which is hereby incorporated by reference in itsentirety. The module 10 includes a frame 11 on a wheeled base 12, whichhas depending therefrom a set of adjustable feet 13 for leveling themodule 10 and anchoring the module 10 to a floor. The module 10 includesa cabinet 14 fixed to the frame 11 that contains flow controllers withconnections for inlet lines for supplying reactant gases to a chemicalvapor deposition (CVD) reactor 15, also fixed to the frame 11. Thecabinet 14 has associated with it other parts of the reactor supportsystem that are not shown, including fluid conduits, valves, pumps,controls, and associated hardware for the operation of the reactor 15including the supplies and connections to supplies of the variousreactant gases, inert gases, purging and cleaning gases, and coolingfluids for the reactor 15.

The reactant gases for the main CVD process to be performed with thereactor 15 are gases used for a blanket tungsten deposition process ontosilicon semiconductor wafers and are supplied through lines 16, shown asfour in number, connected between the cabinet 14 and the reactor 15.These gases include, for example, tungsten hexafluoride (WF₆), hydrogen(H₂), and silane (SiH₄). The reactor is, however, also useful fortitanium nitride films and for many other films that can be appliedthrough a CVD process. Also supplied through one of the lines 16 may beinert gas such as argon. In addition, reactant gas for the plasmacleaning of the chamber 15, such as nitrogen trifluoride (NF₃) gas, issupplied through a gas inlet line 17 connected between the cabinet 14and the reactor 15. The module 10 also includes one or more vacuum pumps18, and usually one high volume low vacuum pump and one low volume highvacuum pump, for evacuating the reactor 15, for maintaining a vacuumwithin the reactor 15 at the required operating pressure levels, and forexhausting unused reactant gas, reaction byproducts, cleaning gases andinert gases flowing through the reactor. A residual gas analyzer port 19is provided for monitoring the constituents of the gas.

The reactor 15 includes a susceptor rotating and wafer elevatingmechanism 20 depending from the bottom of the reactor 15. The mainevacuation of the reactor 15 is accomplished through a vacuum outletline 21 connected between the reactor 15 and the vacuum pump or pumpassembly 18 while one or more auxiliary vacuum outlet lines 22a, 22b areprovided, connected between the mechanism 20 and the pump assembly 18. Acombined upper electrode electrical terminal and cooling fluid manifoldconnector 23 and a combined lower electrode electrical terminal andcleaning gas connector 24 are also connected between the reactor 15 andthe support systems in the vicinity of cabinet 14.

Referring to FIG. 2, the CVD reactor 15 has sealed within it a reactionchamber 25 enclosed in a housing 26 by which the reactor 15 is mountedthrough rubber vibration absorbing pads 29 to the frame 11 and from thebottom of which the mechanism 20 is supported. The housing 26 ispreferably made of aluminum with a highly polished interior, and isprovided with independent temperature control, both for heating andcooling of the reactor walls, to produce what is sometimes genericallyreferred to as a cold wall reactor, as distinguished from an oven typereactor in which the susceptor is heated by radiant heat from a heatedreactor wall. The housing 26 is preferably fluid cooled, by a suitablefluid such as ethylene glycol or water. In addition, resistance heatingelements (not shown) are also provided in the housing 26 so that thehousing may be heated, or, alternatively or in addition, rod typeheating elements may be provided in the chamber at various locations.One or more of the heating or cooling features may be employed in thesame structure, depending on its intended applications. The heating andcooling of the reactor wall may be zone controlled, and may have boththe heating and cooling active simultaneously for more responsivetemperature regulation and uniformity.

The housing 26 has, at the top thereof, a chamber cover 27, preferablyalso of aluminum, enclosing the reaction chamber 25 within. The cover 27is pneumatically sealed against the top of the housing 26, or spacers 81a if employed, and may be pneumatically held thereto or may bemechanically secured thereto by screws 28 or clamps. The cover 27 has areactant gas mixing chamber 30 surrounded by an annular mixing chamberwall which may be formed integrally of the aluminum chamber cover 27 orof a separate material such as a machinable ceramic or separate aluminumor other metal piece and secured to the underside of the chamber cover27. The mixing chamber wall 31 is capable of being actively cooled,where the process, for example a tungsten deposition process, sorequires, by cooling fluid supplied to flow through an annular passage32 formed in the wall 31 to maintain it at a temperature lower than thereaction temperature that is independent of that of the housing 26 andthat of the chamber cover 27. Like the housing 26, the mixing chamberwall 31 is also provided with resistance heating elements (not shown) toheat the wall and the mixing chamber 30 where the process so requires,such as for titanium nitride deposition. This annular wall 31 may bemade of a thermally nonconductive material or of a conductive materialthermally insulated from the aluminum material of the cover 27 toprovide greater flexibility in the control of its temperature. The upperportion of the mixing chamber 30 is closed by a removable cover or topplate 33, preferably of stainless steel, which is sealably connected tothe chamber cover 27 by bolts (not shown). The chamber housing 26,chamber cover 27 and top plate 33 form a sealed vessel enclosing aninternal volume that is maintained at a vacuum pressure level duringoperation of the module 10.

The bottom of the gas mixing chamber 30 is closed by a circularshowerhead 35 connected to the bottom of the mixing chamber wall 31. Theshowerhead 35 may be made of aluminum or of a machinable ceramicmaterial and has a highly polished lower surface to retard theabsorption of radiant heat from the higher reaction temperature from thearea of a wafer being processed within the chamber 25. The showerhead 35may have a uniform pattern of holes therethrough (not shown), preferablyarranged in a matrix or an array in plural concentric circles about thecenter thereof, which lies on a vertical axis 37 through the reactor 15.Alternatively, the showerhead 35 may be formed of a porous metal orceramic plate.

A plurality of gas inlet ports (not shown) are provided in the top plate33 to which the gas lines 16 are connected. A rotary wafer supportingsusceptor 40 is provided within the reaction chamber 25. The susceptor40 lies on the axis 37 directly beneath the showerhead 35 and is inaxial alignment therewith. A cleaning gas entry port 41 is mounted tothe chamber cover 27 and is connected to the cleaning gas input line 17.The RF upper electrode terminal and cooling water connector 23 is alsomounted to the chamber cover 27. The lower electrode RF terminal andcleaning gas connector 24 are mounted to the side wall of the housing26. A single vacuum outlet port 42 is provided in the bottom of thechamber housing 26 to which the vacuum outlet line 21 is connected tothe pump 18, which operates at a pumping rate of from 400-500 liters persecond to achieve the wafer processing pressures at between 1 and 100Torr, reactor cleaning pressures of from 0.1 to 100 m Torr, and wafertransfer pressures of 10⁻⁴ Torr within the chamber 25. A gate port 43 isprovided in the forward wall of the housing 26 for connection to atransport module or wafer handling module of a cluster tool, to and fromwhich wafers are loaded and unloaded from chamber 25 for processing. Thegate 43 is approximately in horizontal alignment with an upwardly facingwafer supporting top surface 44 of the susceptor 40 whereupon a wafer issupported for processing with its upwardly facing side disposedhorizontally parallel to and in vertical alignment with the showerhead35. A plurality of ports 45 are provided in horizontal alignment withthe wafer support surface 44 or the housing 26 on opposite sides of thereaction chamber 25 for inserting diagnostic or other instrumentation.

Fixed to the bottom of the housing 26 and aligned with the reactor axis37 is a susceptor drive support frame 47. Rotatably mounted within thedrive support frame 47 is a hollow and circular in cross-sectionsusceptor drive shaft 50. The drive shaft 50 is mounted to rotate on itsaxis, which is on the reactor axis 37, extends through a hole 51 in thebottom of the reactor housing 26, and is rigidly connected to the bottomof the susceptor 40. At the hole 51, the shaft 50 is rotatably supportedon a main bearing 52 having its inner race surrounding the shaft 50 intight contact therewith and its outer race fixed to the frame 47 at thebottom of the housing 26. A secondary bearing 53, connected to the lowerend of the frame 47, tightly surrounds and supports the lower end of thedrive shaft 50. Secured to the support frame 47 immediately below thebearing 52 and tightly surrounding the shaft 50 is a ferrofluidic seal54. The ferrofluidic seal 54 has fluid circulated through it at atemperature of less than 70° C. to prevent the ferrofluid within it fromdecomposing and losing its magnetic properties due to heat from theshaft 50. Above the secondary bearing 53 within the frame 47 and alsosurrounding the shaft 50 is an electrical slip ring connector 55. Theslip ring 55 provides electrical connection with the rotating shaft 50to supply electrical energy to the rotating susceptor and receivessensed temperature signals therefrom. Fixed to the shaft 50 between theseal 54 and the slip ring 55 is a drive pulley 56 which is drivablyconnected through a drive belt 57 with the output of a susceptorrotation drive motor 58.

At the lower end of the rotating and elevating mechanism 20, fixed tothe bottom of the frame 47, is a wafer lift mechanism 60, illustrated inmore detail in FIG. 3. The lift mechanism 60 includes an outerfluid-tight shell 61 with a hollow interior enclosing the lower end of ahollow and vertical lift tube 62. The tube 62 extends vertically fromthe lift mechanism 60 upwardly through the frame 47 and through thehollow interior of the drive shaft 50, along the axis 37 of the reactor,and into the chamber 25, terminating in the interior of the susceptor 40as may be seen in FIGS. 4 and 5. The tube 62 rotates with the driveshaft 50 and slides axially therein a distance of approximately ninemillimeters to raise and lower a wafer on the wafer support surface 44of the susceptor 40 in the reaction chamber 25. The lower end of thetube 62 is fixed to a hub piece 63 and rotatably supported in aferrofluidic seal 64, the outer surface of which is fixed in a sleeve 65which is vertically slidable in the shell 61. The lower end of thesleeve 65 is linked to a vertical actuator 66 extending through a hole67 in the bottom of the shell 61 of a linear action pneumatic lift 66a.Another ferrofluidic seal 68 is provided near the upper portion of theinterior of the shell 61 surrounding the tube 62 on the axis 37 adjacentthe bottom of the frame 47 of the rotating and elevating mechanism 20.As with the ferrofluidic seal 54, the seals 64 and 68 are supplied withfluid that is maintained at a temperature of 70° C. or less.

An inlet port 70 at the bottom of the shell 61 of the lift mechanism 60communicates with inlet channel 71 at the base of the hub piece 63,which communicates through the hollow interior thereof with an axialbore 72 of the tube 62, extending the length thereof, to communicatewith the susceptor backplane as discussed below.

A vacuum outlet port 74 is provided in the shell 61 and connects with anelongated hollow tube 73 to apply vacuum in a hollow space 75 within thedrive shaft 50 at the upper end thereof surrounding the tube 62, asillustrated in FIG. 3A. The hollow space 75 extends the length of thedrive shaft 50 and also communicates with the interior of the susceptor40 within the reaction chamber 25 as illustrated in FIG. 4.

The vacuum provided in the interior of the susceptor creates a firstvacuum environment which surrounds and protects the heating elements ofthe susceptor as discussed in greater detail hereinbelow. This vacuumpressure is communicated between the vacuum port 74 and the space 75 atthe top of the drive shaft 50 through an annular column or space 79 thatsurrounds the tube 62 and lies within the tube 73.

Vacuum pump 18 is connected to port 74 through line 22a and is connectedto port 70 through line 22b to create first and second vacuumenvironments as will be described. Appropriate valving mechanisms 90aand 90b in lines 22a, 22b, respectively, control the vacuum pressures atports 74 and 70. Furthermore, gauges 91a, 91b are used to measure thevacuum pressures in lines 22a, 22b. Gauges 91a, 91b are coupled to flowcontrollers 92a, 92b, respectively, for controlling helium flow from ahelium supply 94 which supplies helium to both ports 70, 74 throughlines 96b and 96a, respectively. A vacuum exists at ports 70, 74 thesame time that helium gas is being introduced into the ports.

Referring now to FIG. 4, susceptor 40 includes a susceptor body 100which is connected to a generally planer susceptor backplane 102 such aswith screws 103. Preferably, around twenty such screws are placed aroundthe periphery of the susceptor backplane 102 to firmly and evenly securethe backplane to body 100 and are recessed into backplane 102. Anannular Nickel cap 105 surrounds backplane 102 and covers screws 103 topresent a flat backplane surface 104. The susceptor body 100 ispreferably formed of a nickel-copper alloy such as Monel 400 availablefrom Pinnacle Manufacturing of Phoenix, Ariz. Susceptor backplane 102 isnickel. The backplane 102 includes a wafer supporting surface 104 whichsupports a wafer 106 thereon. The bottom portion of body 100 includes acollar portion with a generally horizontally extending annular flange110. Flange 110 sits on top of a shoulder surface 112 formed at the topof the drive shaft 50 and screws 113 extend through flange 110 and intodrive shaft 50 proximate the shoulder surface 112 to secure thesusceptor body 100 to shaft 50 so that the susceptor rotates when shaft50 rotates. A sleeve 114 surrounds shaft 50 proximate susceptor 40 andis mounted to the floor of housing 26.

In the bottom surface of susceptor backplane 102, an annular recess 118is formed to receive a circular resistive heating element 120. Heatingelement 120 is held within recess 118 preferably in intimatesurface-to-surface contact with the susceptor backplane 102. Heatingelement 120 is held against backplane 102 by a ceramic backing plate 122which is secured to the bottom surface of backplane 102 by screws 124(only one shown). Heating element 120 is electrically connected to anappropriate control line 126 which extends radially inwardly to thecenter of the susceptor 40 and then downwardly parallel with the centeraxis 37 to connect with control circuitry (not shown) for controllingthe temperature of the heating element 120 and the backplane 102 so thata wafer 106 supported on the backplane may be heated according toprocess parameters. Line 126 is connected to heating element 120 throughtwo spring loaded connectors 127 (one of which is shown) which sitswithin a recess in the ceramic backing plate 122. A thermal sensor suchas a thermocouple 128 is connected to the heating element 120 throughceramic backing plate 122. Thermocouple 128 is attached to an associatedsensor line 130 which extends downwardly through the center of thesusceptor 40 parallel axis 37 to appropriate heater control circuitryand provides feedback to the control circuitry such that the temperatureof the heating element 120 may be thereby adjusted through line 126 forproper heating of the wafer 106. Lines 126 and 130 make electricalconnection through slip ring 55 (See FIG. 2) to the power supplies andcontrol circuits.

A hub 132 made of ceramic macor encircles the center of susceptor 40inside the susceptor body 100 and includes a radial portion 134 whichrests on a bottom surface of susceptor body 100 inside of the susceptorbody. The top of hub 132 abuts against the bottom surface of the ceramicbacking plate 122 inside of a sleeve 136 depending from the bottomsurface of the ceramic backing plate 122. The bottom of hub 132, inturn, abuts against a plastic sleeve 138 which extends inside driveshaft 50 concentrically therewith and surrounding lift tube 62. Sleeve138 aligns heater and thermocouple lines 126, 130 with the center axis37 of the susceptor.

The susceptor body 100 forms a heater housing volume 140 therein whichgenerally surrounds the heating components of susceptor 40 includingheating element 120 and electrical connector 127 and line 126, as wellas thermocouple 128 and line 130. Volume 140 is in communication withspace 75 formed by drive shaft 50 and thus is vacuumed when space 75 isvacuumed such as by vacuum outlet line 22a and pump assembly 18. Inaccordance with the principles of the present invention, heater housingvolume 140 within susceptor body 100 is sealed to be fluid and air tightto protect the heating components, and particularly the heating element120, from the corrosive chemical vapors within chamber 25.

More specifically, susceptor body 100 is sealed to the backplane 102 andshaft 50 with sealing structures. A top annular seal 142 is positionedwithin an annular channel 143 formed at the interface between thesusceptor body 100 and the backplane 102 generally around the peripheryof backplane 102 as illustrated in FIG. 4A in greater detail. Seal 142is preferably a silver compression seal made of 100% silver having agenerally rectangular transverse cross-section. The width W of seal 142is preferably 3/16 inch in the embodiment shown in the Figures, whilethe thickness T is preferably 1/16 inch for that embodiment. Thediameter of seal 142 will be dependent upon the size of the susceptor 40and the placement of the channel 143 in susceptor body 100 and backplane102. A suitable seal 142 for the embodiment in the Figures ismanufactured by Pinnacle Manufacturing in Phoenix, Ariz. As may beappreciated, the dimensions of the annular seal 142 may be varieddepending upon the construction of the susceptor.

The interface of backplane 102 at channel 143 includes a sharply angledannular ridge 146 which extends vertically downward and presses into thesoft metallic seal 142 when backplane 102 is secured to the susceptorbody 100 (See FIG. 4A). The susceptor body 100 includes a similar angledridge 148, opposite ridge 146, which extends vertically upward into seal142. The ridges 146, 148 and seal 142 insure that a gas-tight seal ismade at the interface between the susceptor backplane 102 and body 100to isolate volume 140 from the reaction chamber 25 and prevent entry ofcorrosive reactant gases into volume 140 through the interface.

The heater housing volume 140 is further sealed by a bottom annular seal150 around the interface between the susceptor body 100 and shaft 50.One useful seal for both annular seal 150 is a composite metal sealformed in accordance with U.S. patent application Ser. No. 08/241,192filed May 11, 1994 entitled THERMAL CYCLE RESISTANT SEAL FOR USE INSEMI-CONDUCTOR WAFER PROCESSING APPARATUS owned by Materials ResearchCorporation and which is incorporated entirely herein by reference.Referring now to FIG. 6, an appropriate sealing element or seal 150 isshown enlarged. Seal 150 is circular in the form of a ring and includesa rigid metallic core 152 and a ductile metallic coating 154 on the core152. Preferably the rigid metallic core 152 is stainless steel, and theductile metallic coating 154 is silver. As is seen in FIG. 6, thecross-section of the seal 150 is generally diamond-shaped. Thisdiamond-shaped seal cross-section has upper and lower blunt tips 156 and158, respectively. The clamping force generated by screws 113,preferably stainless steel screws, of the flange 110, or upper gland,and the shaft 50, or lower gland, on the seal 150 preferably develops acontact stress in the ductile coating 154 sufficient to plasticly deformthe ductile coating 154 to accommodate surface imperfections on thesusceptor and shaft sealing surfaces 160 and 162, respectively, butinsufficient to cause ultimate failure of the ductile coating 154.

In the preferred form of the seal 150 of the present invention, the core152 has the following dimensions prior to being coated by coating 152:The seal core cross-section has a dimension parallel the seallongitudinal axis of symmetry of about 0.105 inch to about 0.107 inch.Further, the seal core has an inner diameter of about 2.645 in., and anouter diameter of about 2.865 in., and therefore the seal corecross-section has a dimension transverse the seal longitudinal axis ofsymmetry of about 0.110 in. Measuring from the radially innermost edgeof the blunt tips, the seal core cross-section has a diameter of about2.743 in., and measuring from the radially outermost edge of the blunttips the seal core cross-section has a diameter of about 2.767 in. Thus,the upper and lower blunt tips 156, 158 of the seal core cross-sectionhave a dimension transverse the seal longitudinal axis of about 0.012in. The seal core cross-section includes four angled seal cross-sectionfaces 164, 166, 168 and 170 each of which forms an angle of about 50°with respect to the seal longitudinal axis of symmetry. The metallicductile coating 154 applied to core 152 preferably has a thickness ofabout 0.003 to about 0.005 in.

The stainless steel utilized in core 152 is preferably 17-4 stainlesssteel heat treated to condition H-1100. The abovementioned dimensionallimits apply prior to silver plating of the seal 150. The silver platingis to be applied per QQ-S-365 (fully annealed) to a thickness of, asspecified above, about 0.003 inch to about 0.005 inch and uniform within0.001 in. After plating, the seal 150 is to be baked to 375° F. forthree hours to remove hydrogen embrittlement from the plating. Afterplating, the dimension of the seal cross-section parallel the seallongitudinal axis of symmetry should be within about 0.111 inch to about0.117 inch.

The seal 150 thus fabricated maintains a leak tight seal after repeatedthermal cycling. The seal 150 has successfully operated at 510° C.through two thermal cycles with no signs of leakage. The seal 150therefore is constructed of a rigid metallic core material with aductile coating that will plasticly deform to accommodate the surfaceimperfections that exist on the sealing faces of the gland, such as thesurfaces 160, 162 of the susceptor body 100 and shaft 50. The dimensionsof the core material and ductile coating must be large enough to ensurethat the total seal thickness, controls the separation distance betweenthe sealing faces of the gland. Furthermore the seal clamping elements(typically screws or bolts) must provide sufficient elasticity at theoperating temperatures to accommodate any thermally induced dimensionalchanges and maintain sufficient contact pressure at the sealing faces ofthe gland at all times.

The material of the clamping elements, glands and seal core are selectedto provide the correct combinations of coefficient of thermal expansionto minimize the change in the gland seal face separation distancethroughout the operating temperature range. The susceptor drive shaft 50is preferably fabricated of 17-4 PH stainless steel, and the susceptorbody 100 is preferably fabricated of Monel 400, a copper and nickelbased alloy as already discussed. The clamping element material anddesign are selected to allow the elements to maintain clamping of thegland seal faces throughout the operating temperature range. Seal 150 isdiscussed in greater detail in U.S. patent application Ser. No.08/241,192 referenced above.

Another appropriate seal structure for sealing the interface betweensusceptor body 100 and shaft 50 is the Helicoflex seal type HNV 230/PartNo. H-303183 from Helicoflex Components Division of Columbia, S.C.Referring to FIG. 7, the seal 200 includes a spiral alloy spring 202which is coiled with the coils welded together to form a generallyunitary structure. An intermediate layer 204 of Inconel 600 surroundsspring 202. Intermediate layer 204 has opposing angled ends 205, 206 anddoes not extend entirely around the spring 202. On the outside of seal200, a layer of nickel 208 surrounds spring 202 and intermediate layer204. Nickel layer 208 has tapered ends 209, 210 and the nickel layer 208is wound around spring 202 and intermediate layer 204 such that ends209, 210 overlap at 211. At the top and bottom surfaces of the seal areridges 212, 214 which are preferably formed with a point. The ridgesembed into the respective surfaces of the interface between body 100 andshaft 50. Ridge 214 is shown embedded into surface 216. Seal 200operates preferably around 450° C. maximum and is compressed with aseating load of approximately 1150 pounds/inch. The interface surfacesare preferably machined to a groove finish of less than or equal toapproximately 32 micro inches rms, while the hardness at the interfaceshould preferably be greater than or equal to approximately 150 HV onthe Vickers Hardness Scale.

The Helicoflex seal 200 has a generally circular cross-section having apreferable cross-sectional dimension of 0.109 inches. The nickel of seal200 allows it to withstand repeated thermal cycling. which is able towithstand repeated thermal cycling. The ring diameter of the annularseal 150 or 200 for the embodiment shown in the Figures is approximately2.75 inches; however, the ring diameter and cross-sectional dimensionsof the seal will generally be dependent upon the susceptor dimensions.

Seals 142 and 150/200 provide a heater housing volume 140 around theheating element 120 which is generally gas-tight to protect the heatingelement. Volume 140 communicates with vacuum space 75 and is thusevacuated to the pressure maintained with the drive shaft 50 and space75. Preferably, volume 140 is maintained at a pressure which is the sameas or slightly higher than the pressure maintained within reactionchamber 25. In that way, any leakage between volume 140 and chamber 25will be out of the susceptor 40 and volume 140 rather than into volume140. This insures that the corrosive reactant chemical gases and vaporswhich are utilized in the CVD processes within chamber 25 do not entervolume 140 and corrode or otherwise damage heating element 120. Thesusceptor body 100 and seals 142, 150/200 form a first vacuumenvironment within volume 140 which is maintained at a pressure in therange of 1 Torr to 100 Torr depending on the process pressure within thechamber 25. The preferred internal pressure of volume 140 isapproximately 25 Torr. The seals 142 and 150/200 are high temperaturemetal seals which are able to withstand the elevated temperaturesutilized in CVD processing and are thus functional after several or morethermal cycles.

When volume 140 is vacuumed, helium is also introduced into the firstvacuum environment. Referring again to FIG. 3, a supply of helium 94 iscoupled to vacuum line 22a by line 96a. An appropriate valve device 90adetermines the internal vacuum pressure in line 22a and the pressure atport 74, which, in turn, sets the pressure in space 75, and ultimately,in volume 140 (see FIG. 4). Gauge 91 a measures the internal pressure ofvolume 140. In one embodiment of the invention, Gauge 91a is preferablya capacitance manometer, and a suitable manometer for the presentinvention is the MKS model #122A-00100DB available from MKS Instrumentsin Andover, Mass.

After a suitable pressure has been produced in volume 140, a helium flowcontrol device 92a, such as MKS model 1159B-00100RV-S mass flowcontroller available from MKS Instruments of Andover, Mass., introduceshelium into space 75 and volume 140. A control line 97a couples gauge96a to mass flow controller 92a to control the introduction of helium.The helium surrounds and protects the heating element 120 in susceptor40. In accordance with the principles of the present invention, thevolume 140 is initially evacuated to around 0.1 milliTorr and then thehelium is introduced raising the pressure to between 1 and 100 Torr andpreferably around 25 Torr. Control line 97a communicates the initialinternal pressure of space 75 to flow controller 92a to control the flowof helium to achieve the desired end pressure.

As discussed hereinabove, for a more efficient heat transfer, a backsidegas is provided to the back of wafer 106 which rests on surface 104 ofthe susceptor backplane 102. Generally, thermal response and heating ofthe wafer 106 is improved when a gaseous heat transfer medium isprovided between the heated susceptor backplane 102 and the backside ofwafer 106. However, as discussed above, in order to provide a passagewayfor backside heating gas, a penetration or opening must be made throughthe susceptor and particularly through the susceptor backplane 102.Generally, such a penetration would provide an opening into volume 140which houses heating element 120. When a wafer is not present over theopening in susceptor backplane 102, there is a direct path from thereaction chamber 25 to volume 140 and the heating element 120, therebyallowing corrosive vapors to reach heating element 120.

In accordance with the principles of the present invention, a second orinner vacuum environment is created within susceptor 40 such thatbackside heating gas may be presented to the wafer 106 without exposingvolume 140 and heating element 120 to corrosive chemical vapors. Theinner vacuum environment is created around hollow lift tube 62 and isisolated from the first vacuum environment in volume 140. Referring toFIGS. 4 and 5, lift tube 62 is hollow along almost its entire length andextends vertically through the center of susceptor 40 within drive shaft50. Lift tube 62 is preferably stainless steel and the top end thereofincludes a solid stainless steel tip 176. Tip 176 connects to a waferlifting assembly 178 which preferably includes approximately threeradial legs 180 with lift pins 182 at the ends thereof which restagainst the backside of wafer 106. (See FIG. 4). Lift tube 62 movesvertically within drive shaft 50 and through an aperture or opening 184formed within backplane 102. As mentioned above, lift tube 62 is movedupwardly and downwardly appropriately 9 mm to raise and lower wafer 106.The tip 176 includes a reduced-diameter portion which moves within astop structure 185 positioned coaxially with opening 184. Stop structure185 includes a cooperating reduced-diameter section which limits theupward movement of tip 176 and tube 62 as may be seen in FIG. 5.

To facilitate a second vacuum environment, lift tube 62 is surrounded byan elongated stainless steel sheath 186 which extends along and covers aportion of tube 62. The top end of sheath 186 includes a flange 188which, in one embodiment of the invention, is fixed to backplane 102with screws 189. Like tube 62 and coaxial therewith, the sheath 186extends through the center of the susceptor 40 and heating element 120and abuts against backplane 102 proximate opening 184. As best seen inFIG. 5, sheath 186 defines a vacuum space 190 between the sheath 186 andouter wall 191 of lift tube 62. A series of apertures 192 are formed inthe lift tube 62. The apertures 192 are within the portion of tube 62covered by the sheath 186 and are preferably arranged radially aroundtube 62. The apertures 192 and tube 62 provide a connection to the boreor space inside hollow tube 62 which is evacuated by pump 18 via lines22b and port 70. This in turn, evacuates vacuum space 190. Apertures 192also provide openings for introducing a backside heating gas, such ashelium, into space 190 for delivery to the wafer 106 through opening184. Backside heating gas introduced by supply 94 and line 96b movesthrough space 190 and through opening 184 to contact the backside ofwafer 106 for efficient thermal conduction between backplane 102 and thewafer. The backside heating gas is distributed around wafer 106 byradial channels 193 and an annular channel 195 connecting the ends ofchannels 193 which are formed in the top surface 104 of backplane 102.Radial channels 193 receive the legs 180 of the wafer lifting assembly.

In accordance with the principles of the present invention, space 190forms a second vacuum environment within the susceptor to isolateheating element 120 from corrosive chemical vapors. Referring to FIG. 5,in one embodiment of the invention, at the top end of sheath 186 anannular upper seal 194 is placed at the interface between backplane 102and a sheath flange 188. Upper seal 194 is preferably a nickelcompression seal made of 100% nickel and available from State Seal inPhoenix, Ariz. The seal 194 extends around the sheath flange backplaneinterface within a channel 197 formed in sheath flange 188. Seal 194forms a gas-tight interface proximate opening 184 such that corrosivechemical vapors of reaction chamber 25 cannot enter volume 140 at theinterface. The seal 194 is compressed at the interface between backplane102 and flange. Seal 194 furthermore isolates the volume 140 from vacuumspace 190 to maintain the integrity of the first and second vacuumenvironments.

In an alternative embodiment of the invention, as shown in FIG. 5A, theflange 188 of sheath 186 is welded to backplane 102 proximate opening184 to seal volume 140 from volume 190 and the environment of chamber25. The welding eliminates the necessity of both screws 189 and seal 194as illustrated in FIG. 5A.

As illustrated and discussed, the sheath 186 defining the second vacuumenvironment surrounds the lift tube 62 which is also surrounded by thespace 75 of shaft 50 which makes up a portion of the first vacuumenvironment. (See FIG. 5). To maintain the integrity of the secondvacuum environment and to isolate it from the first environment alonglift tube 62, the sheath 186 is sealed to the tube. Specifically, at thebottom end of sheath 186 there is a cylindrical flange portion 196 whichhas an annular channel 198 formed therein. Channel 198 faces and abutsagainst the outer wall 191 of tube 62. A lower annular seal 199 isplaced within channel 198 to seal vacuum environment space 190 fromspace 75 which communicates with volume 140 forming the heater housingenvironment or first vacuum environment. The seal 199 is preferably adynamic seal which allows movement of the lift tube 62 therethroughwithout compromising the seal formed at the interface between the wall191 of lift tube 62 and the cylindrical flange portion 196 of sheath186. That is, lift tube 62 slides up and down with respect to seal 199and outer wall 191 moves adjacent seal 199 without deleterious effectson the integrity of the second vacuum environment. A suitable seal forsuch a purpose is the VITON™ seal available from State Seal of Phoenix,Ariz. The VITON™ seal 199 is circular in cross-section and preferablyhas a cross-sectional dimension of 0.070 inches. The diameter of theannular seal is preferably 0.239 inches, although the dimensions of theseal may vary as discussed above.

The vacuum space 190 is formed between flange 188 and seal 194 at theupper end of sheath 186 and flange portion 196 and seal 199 at the lowerend of sheath 186. Apertures 192 for the evacuation of space 190 and theintroduction of a backside heating gas are located in tube 62 betweenseals 194, 200. Space 190 is preferably maintained at a vacuum pressureof approximately 1 Torr to 100 Torr, depending on the process pressurewithin chamber 25. The preferred pressure is 10 Torr. During CVDprocessing, the pressure in the inner space 190 is generally maintainedless than the process pressure within the reaction chamber 25 when awafer 106 is present on surface 104 of the susceptor backplane 102. Thelower pressure within space 190 maintains a vacuum holddown of the wafer106 when the wafer covers the opening 184 and channels 193, 195 withinthe susceptor backplane 102.

When the wafer is vacuum held, the lift tube 62 is at its lowestvertical position, and the lift-assembly 178 is positioned withinchannels 193, 195 formed within backplane 102. When the processing hasbeen completed, the lift tube 62 is raised which raises tip 184 and thelift-pin assembly 178 such that lift pins 182 press against the backsideof wafer 106 and break the vacuum seal so that the wafer can be removedfrom the process chamber (See FIGS. 4 and 5).

The second vacuum environment inside tube 62 and space 190 is createdand maintained very similarly to the way in which the first vacuumenvironment is created and maintained. Referring again to FIG. 3, thevacuum line 22b connects with port 70 and is controlled by the valvingmechanism 90b. The pressure gauge 91b, such as a capacitance manometersimilar to gauge 91a, measures the pressure a port 70 and within thesecond vacuum environment. The inner bore of lift tube 62 and space 190(see FIG. 5) is evacuated to around 0.1 milliTorr. A control line 97bconnects gauge 90b to a mass flow controller 92b similar to mass flowcontroller 92a already described. When the desired pressure in tube 62is achieved as measured by gauge 90b, the mass flow controller isoperated, via line 22b, to inject helium into port 70 which raises thepressure to a pressure of from 1 to 100 Torr, and preferably around 10Torr. The helium travels through lift tube 62, into space 190 andagainst the backside of wafer 106 for efficient heat transfer to thewafer. When a wafer 106 is not present on surface 104, the space 190 isin direct communication with the reaction chamber 25. However, becausespace 190 is isolated from the first vacuum environment in volume 140surrounding the heating element 120, there is no damage to the heatingelement by the corrosive chemical vapors in the reaction chamber 25.

Therefore, the present invention provides complete isolation of anyheating elements and other devices within susceptor 40 and volume 140 toprevent corrosion of those elements. The first and second vacuumenvironments 140, 190 along with the four seals associated therewitheffectively provide isolation of the heating element environment fromthe reaction chamber environment. Simultaneously, the backside heatinggas is introduced through a second vacuum environment to provideefficient thermal transfer of heat from backplane 102 to wafer 106.Seals 142, 150, 194 and 200 are able to withstand the high temperaturesassociated with CVD processing.

While the present invention has been illustrated by the description ofthe embodiments thereof, and while the embodiments have been describedin considerable detail, it is not the intention of applicant to restrictor in any way limit the scope of the appended claims to such detail.Additional advantages and modifications will readily appear to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details, representative apparatus andmethod, and illustrative examples shown and described. Accordingly,departures may be made from such details without departure from thespirit or scope of applicant's general inventive concept.

What is claimed is:
 1. An apparatus for efficiently heating a waferwithin a CVD environment at a process pressure and effectively isolatinga heating element from the CVD environment comprising:a susceptorassembly comprising a susceptor body with a sealed space therein forcontaining the heating element and a surface coupled to the heatingelement for supporting and heating a wafer, the sealed space beingevacuatable at a first pressure to create a first vacuum environmentaround the heating element, and the susceptor body operable toeffectively isolate the first vacuum environment and heating elementfrom the CVD environment; a heating gas delivery space extending throughsaid susceptor body and first vacuum environment and communicating withan opening in said surface to deliver heating gas between a wafer andthe heated surface and facilitate efficient thermal conduction betweenthe surface and the wafer, the delivery space being sealed from saidfirst vacuum environment to effectively prevent corrosive vapors fromentering the first vacuum environment through the gas delivery space;whereby backside heating gas may be supplied to a wafer for efficientheating while the heating element is protected from corrosive chemicalvapors in the CVD environment.
 2. The apparatus of claim 1 furthercomprising a gas delivery tube extending into the gas delivery space todeliver gas thereto and a sealing assembly surrounding a portion of thetube and gas delivery space to seal the tube portion and gas deliveryspace from the first vacuum environment.
 3. The apparatus of claim 2,the sealing assembly including an elongated sheath surrounding the tubeto define the gas delivery space therebetween, the sheath sealed fromsaid first vacuum environment.
 4. The apparatus of claim 3 furthercomprising an annular seal positioned between an end of the sheath andthe susceptor body surface proximate the opening to seal an end of thesheath.
 5. The apparatus of claim 4 further comprising a second annularseal positioned between the other end of the sheath and the tube.
 6. Theapparatus of claim 2 wherein the tube is coupled to a wafer liftingdevice proximate the susceptor body surface, the tube being movable tomove the lifting device and lift a wafer on the surface.
 7. Theapparatus of claim 5 wherein the second annular seal is a dynamic sealwhich allows movement of the tube with respect thereto.
 8. The apparatusof claim 4 wherein the annular seal is a nickel seal.
 9. The apparatusof claim 1 wherein the heating gas delivery space is evacuatable at asecond pressure to create a second vacuum environment in said deliveryspace.
 10. The apparatus of claim 9 wherein the pressure of the secondvacuum environment is less than the process pressure to vacuum hold tothe susceptor surface a wafer positioned over said surface opening, thesecond vacuum environment isolated from the first vacuum environment andthe CVD environment when a wafer is so positioned.
 11. The apparatus ofclaim 1 further comprising a support coupled to the susceptor body, thesusceptor body including a body member and a planar member coupled tothe body member and having said wafer heating surface thereon, a firstsusceptor seal sealing the interface between the planar member and bodymember and a second susceptor seal sealing the interface between thesupport and the body member, the seals facilitating isolation of thefirst vacuum environment from the CVD environment.
 12. The apparatus ofclaim 11 wherein the first susceptor seal is a silver seal.
 13. Theapparatus of claim 11 wherein the second susceptor seal is a compositemetal seal.
 14. The apparatus of claim 11 wherein the support has ahollow space therein which is coupled to the sealed space of the bodysuch that vacuuming the support space creates said first vacuumenvironment in the sealed space.
 15. The apparatus of claim 1 whereinthe pressure in the first vacuum environment is greater than saidprocess pressure such that leaks in the susceptor assembly areeffectively directed out of the first vacuum environment and into theCVD environment.
 16. A susceptor for efficiently heating a wafer withina CVD environment and effectively isolating a heating element of thesusceptor from the CVD environment comprising:a partially hollowsusceptor body mounted on a support shaft and containing the heatingelement; a planar member mounted on the susceptor body and coupled tothe heating element, the planar member having a surface for supportingand heating a wafer; the interface between the susceptor body and thesupport shaft and the interface between the susceptor body and planarmember each being sealed to create a generally vacuum-tight first spacewithin the susceptor body which surrounds the heating element andeffectively protects the heating element from exposure to the CVDenvironment; a gas delivery tube extending through the susceptor bodyand first space to deliver backside heating gas to a wafer placed onsaid planar member surface; a sheath surrounding a portion of said tubeand creating a second space between the tube and sheath, one end of thesheath coupled to an opening in the planar member to direct the backsideheating gas from the tube to the wafer, the sheath sealed to the planarmember at the end coupled to the opening and sealed to the tube at theother end such that the second space is effectively isolated from thefirst space so that backside heating gas may be delivered to the waferto facilitate more efficient heat transfer from the heating elementwhile the heating element remains protected from the corrosive chemicalvapors of the CVD environment.
 17. The susceptor of claim 16 wherein thesleeve is operable to effectively isolate the second space from both thefirst space and the CVD environment when a wafer completely covers theopening in the planar member.
 18. The susceptor of claim 16 wherein theCVD environment is at a process pressure and further comprising a vacuumsystem coupled to the second space to evacuate the second space to apressure less than said process pressure for vacuum holding a wafer overthe planar member opening.
 19. The susceptor of claim 16 wherein thetube extends inside the sheath and is movable with respect to thesheath, the susceptor further comprising a wafer lifting assemblycoupled to the tube and positioned proximate the planar member openingto lift a wafer away from the planar member and opening when the tube ismoved inside said sheath.
 20. A wafer processing susceptor including asurface to support a wafer, a heating element coupled to the surface toheat a wafer on the surface and a backside gas system for delivering gasbetween the wafer and surface to promote thermal transfer from theheating element to the wafer in a CVD reaction space comprising:a firstspace in the susceptor for housing the heating element, the first spacebeing sealed from the CVD reaction space to effectively preventcorrosive chemical vapors in the CVD reaction space from affecting theheating elements; a second space inside of the first space andsurrounding the backside gas system, the second space coupled to anopening in the surface to direct gas to a wafer on the surface forefficient heat transfer between the heating element and the wafer, thesecond space being sealed from the first space to effectively preventcorrosive chemical vapors from entering the first space through thesecond space and affecting the heating elements; whereby backsideheating gas may be supplied to a wafer for efficient heating while theheating element is protected from corrosive chemical vapors in the CVDenvironment.
 21. The susceptor of claim 20 wherein the second space iseffectively sealed from both the first space and the CVD reactions spacewhen a wafer is placed over the surface opening.
 22. A method forefficiently heating, with a heating element, a wafer within a CVDreaction space evacuated to a process pressure while effectivelyisolating the heating element from the CVD environmentcomprising:placing a wafer on a surface of a susceptor, the susceptorhaving a space therein for housing a heating element coupled to thesurface to heat the surface and the wafer; sealing the susceptor spacewith respect to the CVD reaction space to effectively isolate theheating element from the CVD environment; directing backside heating gasinto a space extending through the susceptor space and through anopening in the susceptor surface such that the gas contacts a wafer onsaid surface and facilitates efficient heat transfer between the waferand the heated surface; and sealing the gas directing space with respectto the susceptor space to prevent exposure of the heating element to thegas directing space and to further effectively isolate the heatingelement from the CVD environment; whereby the wafer if efficientlyheated while the heating element is protected from corrosive chemicalvapors existing in the CVD environment.
 23. The method of claim 22further comprising the step of placing a wafer over the surface openingto cover the opening and seal the gas directing space from both thesusceptor space and the CVD reaction space.
 24. The method of claim 23further comprising the step of evacuating the gas directing space to apressure lower than the process pressure to create a vacuum hold of thewafer over the surface opening.
 25. The method of claim 20 furthercomprising the step of maintaining the susceptor space at pressuregreater than the process pressure such that leaks in the sealedsusceptor are effectively directed out of the susceptor space ratherthan into it.
 26. The method of claim 22 wherein the susceptor includesa susceptor body and a planar member having the surface thereon andfurther includes a gas delivery tube extending through said susceptorbody and susceptor space to deliver backside heating gas to the wafer,the method further comprising:surrounding the gas delivery tube with anelongated sheath, the gas directing space being formed between thesheath and the tube; sealing an interface between one end of the sheathand the tube; and sealing an interface between the other end of thesheath and the planar member whereby to seal the gas directing spacefrom the susceptor body and effectively isolate the heating element fromthe CVD environment.
 27. The method of claim 26, the sealing stepsincluding positioning an annular seal structure around the tube end atan interface between the sheath and one of the tube and the planarmember.
 28. The method of claim 27 further comprising compressing anickel seal at the interface between the sheath and the planar member.29. The method of claim 27 further comprising the step of positioning adynamic seal structure at the interface between the tube and sheath toallow movement of the tube within the sheath.
 30. The method of claim 27wherein the gas delivery tube is movable with respect to the susceptorbody, and is coupled to a wafer lifting mechanism, the method furthercomprising lifting the tube to lift the wafer away from the surface. 31.The method of claim 22 wherein the susceptor includes a susceptor body,a support fixed to the bottom of the body, and a planar member fixed tothe top of the body and having said surface, the susceptor space definedby said body, support and planar member and the step of sealing thesusceptor space comprising:sealing an interface between the susceptorbody and support; and sealing an interface between the susceptor bodyand the planar member.
 32. The method of claim 31 wherein the step ofsealing the interface between the susceptor body and planar memberincludes compressing a silver seal at the interface.
 33. The method ofclaim 31 wherein the step of sealing the interface between the susceptorbody and support includes compressing a metal seal having a rigid metalcore and a ductile coating at the interface.
 34. A method of depositinga layer of material on a semiconductor wafer by chemical vapordeposition (CVD) including the step of heating the wafer, the methodcomprising:placing a wafer on a surface of a susceptor, the susceptorhaving a space therein for housing a heating element coupled to thesurface to heat the surface and the wafer; sealing the susceptor spacewith respect to the CVD reaction space to effectively isolate theheating element from the CVD environment; directing backside heating gasinto a space extending through the susceptor space and through anopening in the susceptor surface such that the gas contacts a wafer onsaid surface and facilitates efficient heat transfer between the waferand the heated surface; and sealing the gas directing space with respectto the susceptor space to prevent exposure of the heating element to theCVD reaction space and to further effectively isolate the heatingelement from the CVD environment; introducing CVD reactant gases in theproximity of the wafer to react and deposit a material layer on theheated wafer; whereby a material layer is deposited on a heated waferwhile the heating element is protected from corrosive effects of the CVDreactant gases contributing to the material layer.
 35. The method ofclaim 34 further comprising the step of placing a wafer over the surfaceopening to cover the opening and seal the gas directing space from boththe susceptor space and the CVD reaction space.
 36. The method of claim35 further comprising the step of evacuating the gas directing space toa pressure lower than the process pressure to create a vacuum hold ofthe wafer over the surface opening.
 37. The method of claim 34 furthercomprising the step of maintaining the susceptor space at a pressuregreater than the process pressure such that leaks in the sealedsusceptor are effectively directed out of the susceptor space ratherthan into it.
 38. The method of claim 34 wherein the susceptor includesa susceptor body and a planar member having the surface thereon andfurther includes a gas delivery tube extending through said susceptorbody and susceptor space to deliver backside heating gas to the wafer,the method further comprising:surrounding the gas delivery tube with anelongated sheath, the gas directing space being formed between thesheath and the tube; sealing an interface between one end of the sheathand the tube; and sealing an interface between the other end of thesheath and the planar member whereby to seal the gas directing spacefrom the susceptor body and effectively isolate the heating element fromthe CVD environment.